Japanese Patent No. 3045063 discloses a drive device used for a hybrid electric vehicle, in which one of a first motor/generator and an engine is connected to a sun gear of a planetary gear set, and the other of the first motor/generator and the engine is connected to a planet carrier of the planetary gear set. A second motor/generator is connected to an output shaft of a ring gear. The first motor/generator is mainly used as a generator, while the second motor/generator is mainly used for running the vehicle. The connected planetary gear set functions as a differential mechanism. A portion of engine output is allotted for power generation, and electric power generated by the generator is output from the motor. The planetary gear set thus functions as a continuous variable transmission or a mechanism for increasing and decreasing output torque.